Picture image observing systems are generally of the type wherein a plural number of optical fibers each of which constitutes a picture element are bundled in alignment so that a picture image of an object is focused by means of an image-forming lens at the forward end of the optical fiber bundle, and transmitted through and enlarged by the magnifying lens of an eyepiece or the like at the back end thereof to be inspected. Since the device in such a system is extremely small in diameter and flexible in materials, it has been applied to a wider scope of the fields from industrial applications in various pipes such as heat conductive pipes used in heat exchangers at a chemical plant or a nuclear plant or a steel mill and medical applications to examine inside of a body such as cardiocameras or gastrocameras to civil usages such as security cameras or devices for monitoring dead angles in automobiles. The scope of application will not be limited to the present use but will extend further. One of the viable applications in the future will be to examine atmosphere filled with particles or volatile gases. Unlike conventional TV monitoring systems, this system is completely free of the possibility to trigger electric sparks which might cause an explosion and endanger workers.
However, if the conventional system is used, in which an image-forming lens and the protective filter therefor are mounted without covers on an adaptor on the tip and are passed through the atmosphere of particles, vapor or poisonous gas, they tend to adhere on the surfaces of the image-forming lens and the protective filter, thereby causing deterioration of picture quality and/or chemically damaging the image-forming lens and the protective filter.